1. Field
The present invention relates to a piezoelectric transducer element which is constituted of a piezoelectric body fixed to a movable part and a fixing part in a state where the piezoelectric body extends between and over these portions, and more particularly to a piezoelectric transducer element which reduces an overlapping area of an upper electrode and a lower electrode mounted on a surface of the piezoelectric transducer element, and an actuator, a sensor, an optical scanning device and an optical scanning display device which include the piezoelectric transducer element.
2. Description of the Related Art
Conventionally, there has been known an optical scanning device or an actuator in which a piezoelectric element made of PZT (lead zirconate titanate) or BaTiO3 (barium titanate) is fixed to a base body constituted of a fixing part and a movable part. A conventional optical scanning device 100 is, as shown in FIG. 6A, constituted of a swingable reflection surface 101, first beam portions 102 with elasticity which constitute a swing axis of the reflection surface 101, and second beam portions 103 which support the first beam portions 102, two third beam portions 104 with elasticity which support the second beam portions 103, a fixing frame 105 which fixes the third beam portions 104 thereto, and a support portion 106 which supports the fixing frame 105. The above-mentioned structure has the symmetrical structure with respect to the reflection surface 101. Further, a strain generating element 107 is fixed to an upper portion of the third beam portion 104 in a state where the strain generating element 107 extends between and over the third beam portion 104 which constitutes a movable part and the fixing frame 105 to which the third beam portion 104 is fixed.
As shown in FIG. 6B, the strain generating element 107 is mounted in a state where the strain generating element 107 extends over between the frame-side third beam portion 104 and the fixing frame 105. The strain generating element 107 is constituted of a piezoelectric element 110, an upper electrode 111 and a lower electrode 112 which are formed on both surfaces of the piezoelectric element 110 respectively, input terminals 114 which are formed on the fixing frame 105, and lead wires 113 which electrically connect the upper electrode 111 and the input terminals 114 to each other.
The optical scanning device 100 is driven as follows. When an AC voltage is applied between the upper electrode 111 and the lower electrode 112 of the strain generating element 107, a stress in the vertical direction is imparted to the third beam portion 104 by the strain generating element 107. By applying AC electric fields whose polarities are inverted from each other to two strain generating elements 107, a rotational stress is imparted to the first beam portions 102. Accordingly, the reflection surface 101 swings using the first beam portions 102 as the swing axis. As a result, an incident optical flux which is incident on the reflection surface 101 is scanned when the incident optical flux is reflected and is converted into a scanning optical flux.
Here, the strain generating element 107 of this type usually has an extremely small shape such that a width of the strain generating element 107 is approximately 0.1 mm. Accordingly, a width of the upper electrode 111 which is formed on the piezoelectric element 110 also has an extremely small shape such that a width of the upper electrode 111 is approximately 0.1 mm. The lead wire is wired between the upper electrode 111 and the input terminal 114 formed on the fixing frame 105. A stepped portion formed due to a thickness of the piezoelectric element 110 is present between the input terminal 114 formed on the fixing frame 105 and the upper electrode 111 formed on the piezoelectric element 110. Accordingly, it is difficult to electrically connect an upper surface of the piezoelectric element 110 and the electrode on the fixing frame 105 with each other by a vacuum depositing method such as a sputtering method or a vapor deposition method. In view of the above, the upper electrode 111 formed on the piezoelectric element 110 and the input terminal 114 formed on the fixing frame 105 are electrically connected with each other using the lead wire 113. The lead wires 113 are connected by wire bonding.
In performing the wire bonding, a pressure is applied to a portion to which bonding is applied. When the portion to which the wire bonding is applied is the upper electrode 111 arranged above the third beam portion 104, since the third beam portion 104 has a free end, the pressure cannot be applied to the third beam portion 104. Accordingly, in the above-mentioned related art, the piezoelectric element 110 extends to the fixing frame 105 and is mounted on the upper electrode 111 above the fixing frame 105 by wire bonding.
As shown in FIG. 6C, a pad electrode 115 is formed on the piezoelectric element 110 above the fixing frame 105. The piezoelectric element 110 is formed such that the piezoelectric element 110 extends between and over the third beam portion 104 and the fixing frame 105. In a region of the third beam portion 104, the piezoelectric element 110 is sandwiched between the lower electrode 112 and the upper electrode 111. The lower electrode 112 is formed so as to extend over the surface of the fixing frame 105 and is continuously connected to the input terminal 114b. On an upper surface of the piezoelectric element 110, the upper electrode 111 and the pad electrode 115 which is included in the upper electrode 111 are formed. The pad electrode 115 is formed above the fixing frame 105. The input terminal 114a and the pad electrode 115 are electrically connected with each other by the lead wire 113. Since the pad electrode 115 is formed above the fixing frame 105, it is possible to sufficiently support the pad electrode 115 even when a pressure is applied to the pad electrode 115 in performing the wire bonding of the lead wire 113.
In the above-mentioned optical scanning device 100 of the related art, the lower electrode 112 is formed on the whole lower surface of the piezoelectric element 110. Accordingly, a large electrostatic capacity is generated between the pad electrode 115 and the lower electrode 112. This electrostatic capacity is equal to or larger than an electrostatic capacity in the movable part which is generated between the lower electrode 112 and the upper electrode 111 above the third beam portion 104. As a result, to apply an electric field between the upper electrode 111 and the lower electrode 112 for driving the scanning device, it is necessary to supply an electric current twice or more times as large as an electric current which actually flows in the movable part. Further, to consider a case where the optical scanning device 100 is used as an oscillation sensor, a piezoelectric current generated in the movable part is consumed as an electrostatic capacity in portions other than the movable part and hence, a detection current is reduced. Accordingly, there arises a drawback that a voltage signal taken out from the optical scanning device 100 becomes small or the response characteristics of a signal taken out from the optical scanning device 100 are lowered.
To overcome the above-mentioned drawbacks, according to one aspect of the present invention, there is provided a piezoelectric transducer element which includes: a base body which includes a fixing part and a movable part; a lower electrode which is formed on a surface of the base body; a piezoelectric body which is formed in a stacked manner in a state where the piezoelectric body extends between and over the fixing part and the movable part of the base body; and an upper electrode which is formed on a surface of the piezoelectric body on a side opposite to the base body. The piezoelectric body converts a change in potential between the lower electrode and the upper electrode into mechanical displacement of the movable part relative to the fixing part or converts the mechanical displacement into the change in potential. The upper electrode includes a pad electrode for connection which is formed above the fixing part, and the lower electrode is not formed in a region above the fixing part and below the pad electrode. The piezoelectric transducer element may be used for an actuator, a sensor, an optical scanning device, an optical scanning display device or the like.